When deploying 3rd generation mobile communication technology, different radio access technology systems such as GSM and WCDMA will be available at the same time. From the viewpoint of a multisystem capable mobile station or terminal, both systems can be accessed. However, the network resources are managed by different system networks.
In general, the transfer of a user's connection from one radio channel to another radio channel in the same or a different radio cell of the same or a different cellular network is called handover. Each of the network systems may decide on its own about an intersystem handover (IS-HO) when the mobile is in an active mode. Such an IS-HO is a handover of a connection from a radio channel of the one system, e.g. the WCDMA system, to a radio channel of another system, e.g. the GSM system. One of the key requirements for a successful IS-HO is enough free capacity in the target cell. However, no appropriate signaling for load information exchange might be provided among the network elements initiating an IS-HO. If the need for a handover is detected in one of the systems due to a high network load, an IS-HO will be tried without exact load information about the target cell. This leads to a risk of failure or getting the mobile immediately back due to a corresponding high load situation in the other system (ping-pong effect).
Additionally, in the case of multivendor networks, less information exchange is guaranteed, as e.g. one vendor might group an IS-HO differently into the predefined classes for handover reasons than another vendor. Hence, the different system networks must be able to provide an IS-HO function without being directly informed about the load in the respective other system or systems. However, without information about the load of a target cell, unnecessary IS-HOs might be initiated possibly including compressed mode measurements of mobile terminals for IS-HO from the WCDMA system to the GSM system with additional network capacity consumption, signaling for preparing an IS-HO, and additional processing capacity required in different network elements. This leads to a waste of available radio resources if the load in the target cell does not permit IS-HOs. In addition, IS-HOs between highly loaded cells result in a possible QoS (Quality of Service) deterioration for the connected mobile terminal due to the ping-pong effect and additional and unnecessary risk of loosing the connection.
A 3rd generation's WCDMA system could be operated in principle with a single carrier only. However in the WCMDA system, more than one carrier, may be used. Therefore, handover strategies and algorithms are needed to direct the users in the most efficient way between those carriers. Thus, whenever the WCDMA is not able to serve a user due to an overload in one carrier of the WCDMA system, this user may be handed over to either another carrier in the WCDMA system or to the GSM system. A handover within WCDMA between different carriers is called inter-frequency handover (IF-HO). Such an IF-HO requires the possibility for the mobile terminal to carry out a cell search on a carrier frequency different from the current one, without effecting the ordinary dataflow. The WCDMA system supports inter-frequency cell search in two different ways, a dual-receiver approach and a slotted-downlink-transmission approach. In the dual-receiver approach, a receiver diversity is provided in the mobile terminal, wherein one of the receiver branches can be fully allocated or temporarily reallocated from diversity reception and instead carry out reception on a different carrier. In the case of a slotted-downlink-transmission, a single-receiver mobile terminal is arranged to carry out measurements on other frequencies without effecting the ordinary dataflow. In the slotted mode, the information normally transmitted during a signal frame is compressed in time, either by code puncturing or by reducing the spreading factor e.g. by a factor two. In this way, a time period of e.g. half of the frame period is created during which the receiver of the mobile terminal is idle and can be used for interfrequency measurements. Once initiated such slotted frames may occur periodically, wherein the rate of slotted frames is variable and depends on the environment and the measurement requirements.
The WCDMA technology can offer services which cannot be supported by current GSM systems. Therefore, it could be desirable that users which have a service that can be offered by the GSM system as well, should be handed over to it in order to free capacity for such services that can only be served by the WCDMA system. Moreover, if a mobile terminal is using a service in the GSM system and wants to establish a service which cannot be provided in the GSM system, it should be handed over to the WCDMA system.
Therefore, handovers triggered by certain service and/or load criteria should be provided from the GSM system to the WCDMA system and within the carriers of the WCDMA system to thereby provide a function for sharing users according to current network loads and used services.